1. Field of the Invention
The present invention relates to a process for preparing a 1,1,1-trihalogeno-4-methyl-3-penten-2-ol of the formula [I]: ##STR1## wherein each X represents Cl or Br, comprising thermally isomerizing a 1,1,1-trihalogeno-4-methyl-4-penten-2-ol of the formula [II]: ##STR2## wherein each X is as defined above.
The topic thermal isomerization, conducted at a temperature in the range of from about 100.degree. C. to about 250.degree. C., is advantageously, albeit not necessarily, carried out and/or promoted in the presence of an acid catalyst, a transition metal catalyst, or a transition metal compound catalyst. In the event the thermal isomerization is acid catalyzed, the reaction can be conducted at a temperature in the range of between 50.degree. C. and 250.degree. C.
2. Description of the Prior Art
The 1,1,1-trihalogeno-4-methyl-3-penten-2-ol [I] obtained by the process of the present invention can be utilized for a variety of purposes. For example, 1,1,1-trichloro-4-methyl-3-penten-2-ol is a useful synergistic agent for herbicides [see U.S.S.R. Pat. No. 227,012] and is also known to be physiologically active. In addition, the 1,1,1-trihalogeno-4-methyl-3-penten-2-ol [I] has utility as a starting material in the synthesis of acid components of 2,2-dimethyl-3-(2',2'-dihalogenovinyl) cyclopropane carboxylic esters which exhibit remarkable insecticidal action [see published Japanese Patent Application, Ser. No. 47531/1974, corresponding to British patent specification No. 1,413,491, and D. G. Brown et al., J. Agr. Food Chem., 21, No. 5,767 (1973)]. 2,2-Dimethyl-3-(2',2'-dihalogenovinyl) cyclopropane carboxylic esters can be synthesized by reacting the 1,1,1-trihalogeno-4-methyl-3-penten-2-ols with orthoacetic esters or ketone acetals with or without the aid of an acid catalyst to obtain 3,3-dimethyl-4,6,6 -trihalogeno-5-hexenoic esters and then treating such esters with a basic reagent. According to this process, cyclopropane carboxylic acid esters can be prepared more easily and at a lower cost as compared to those produced by known processes.
1,1,1-Trichloro-4-methyl-3-penten-2-ol is a known compound and can be synthesized by Grignard reaction as follows [J. Chem. Soc., (C), 670 (1966)]: ##STR3## wherein X represents Cl or Br, and ##STR4## represents a ##STR5## group or a ##STR6## group. However, this conventional process has drawbacks in that the Grignard compounds used as starting materials cannot be prepared easily and are needed in stoichiometric amounts which are substantial and hence not economical.
Although Colonge et al report [Soc., chem de France, Bull, 204-208 (1957)].sup.1, that isobutene reacts with chloral in the presence of aluminum chloride at -5-+10.degree., forming a mixture of 30% 1,1,1-trichloro-4-methyl-4-penten-2-ol and 70% 1,1,1-trichloro-4-methyl-3-penten-2-ol, it has been subsequently proven by Klimova et al.sup.2 that Colonge et al erroneously identified the structure of the adduct they obtained and that isobutene reacts with chloral in the presence of aluminum chloride at 0.degree. to selectively form 1,1,1-trichloro-4-methyl-4-penten-2-ol. If 1,1,1-trichloro-4-methyl-4-penten-2-ol could be easily converted into 1,1,1-trichloro-4-methyl-3-penten-2-ol, the process would be economically superior to the aforementioned Grignard process in preparing said 1,1,1-trichloro-4-methyl-3-penten-2-ol. FNT .sup.1 Cf. Colonge et al, C.R., 541-543 (1954) FNT .sup.2 Klimova et al, J. Org. Chem., U.S.S.R., 1308-1311 (1969)
Although there are many reports concerning the migration of double bonds in olefins, it is true, as Yanovskaya et al reported [Russian Chemical Review, 39 (10) (1970)] that the migration of double bonds in olefins is not directly dependent upon the type of the olefin, the situation of any substituent on the olefin and the nature of any solvent employed. For example, in the case of unsubstituted nitro-olefins or derivatives substituted only in the .alpha.-position, the .beta.,.gamma.-isomer is wholly displaced towards the .alpha.,.beta.-isomer, ##STR7## whereas in the case of nitro-olefins substituted in the .beta.- or .gamma.-positions, the .beta.,.gamma.-isomer is not completely displaced towards the .alpha.,.beta.-isomer so as to obtain an equilibrium mixture of major amounts of .alpha.,.beta.-isomer and appreciable amounts of the .beta.,.gamma.-isomer. Moreover, unsubstituted vinylnitriles or derivatives with substituents only in the .alpha.- or .beta.-positions do not isomerize to .beta.,.gamma.-isomer from .alpha.,.beta.-isomer. ##STR8## When encountering two methyl groups or electron-accepting substituents (C.sub.6 H.sub.5, C.sub.6 H.sub.5 O) in the .gamma.-positions, the .alpha.,.beta.-isomer is displaced towards the .beta.,.gamma.-isomer. However, the .gamma.-substituted vinylnitrile does not isomerize where there is a methyl group in the .alpha.-position.
Furthermore, 3-methyl-3-buten-1-ol, which has a molecular structure very similar to 1,1,1-trihalogeno-4-methyl-4-penten-2-ol, is only slight isomerized to 3-methyl-2-buten-1-ol under heat in both an uncatalyzed reaction and one employing p-toluenesulfonic acid [compare Reference Examples 14 and 15 described hereinafter].
Thus, there exists a need in the art for a simple and inexpensive method of isomerizing 1,1,1-trihalogeno-4-methyl-4-penten-2-ol to 1,1,1-trihalogeno-4-methyl-3-penten-2-ol with high selectivity for the latter.